The present invention generally relates to tube suspension systems for magnets and, more particularly, is concerned with a laminated composite shell assembly with enhanced thermal insulating and efficient load distributing properties for enabling use in magnet applications such as in superconductive magnets operating at cryogenic temperatures.
Superconductive magnets include superconductive coils which generate uniform and high strength magnetic fields, such as used, without limitation, in magnetic resonance imaging (MRI) systems employed in the field of medical diagnostics. The superconductive coils of the magnet typically are enclosed in a cryogenic vessel surrounded by a vacuum enclosure and insulated by a thermal shield interposed therebetween.
Various designs of tube suspensions are employed to support the cryogenic vessel of a superconductive coil assembly of the magnet from and in spaced apart relation to both the thermal shield and the vacuum enclosure of the magnet. As one example, the tube suspension can include overlapped fiberglass outer and inner support cylinders, such as disclosed in U.S. Pat. No. 5,530,413 to Minas et al. which is assigned to the same assignee as the present invention. In the Minas et al. tube suspension, the outer support cylinder is located within and generally spaced apart from the vacuum enclosure and positioned outside of and generally spaced apart from the thermal shield. A first end of the outer support cylinder is rigidly connected to the vacuum enclosure while a second end of the outer support cylinder is rigidly connected to the thermal shield. The inner support cylinder is located within and generally spaced apart from the thermal shield and is positioned outside of and generally spaced apart from the superconductive coil assembly. The inner support cylinder has a first end rigidly connected to the thermal shield near the second end of the outer support cylinder and has a second end located longitudinally between the first and second ends of the outer support cylinder and rigidly connected to the superconductive coil assembly.
Problems can occur, however, with some designs of tube suspension systems at cryogenic temperatures. For instance, tube suspensions of some current superconductive magnet designs in MRI systems employ metal alloys or glass-epoxy materials. Metal alloys as well as glass-epoxy materials do not provide optimal load distributing and thermal insulating characteristics. Further, metal alloys are heavy and glass-epoxy materials deform as they tend to be compliant.
Consequently, a need exists for innovation with respect to tube suspensions for supporting superconductive magnets which will provide a solution to the aforementioned problems.
The present invention provides a uniform stiffness laminated composite shell assembly designed to satisfy the aforementioned need. The uniform stiffness composite shell assembly of the present invention has a sequence of composite shells with different laminates having different stacking sequences and employing graphite-epoxy material that provide enhanced thermal insulating and efficient load distributing properties for enabling use in cryogenic applications. The sequence of composite shells provides the combination of properties needed for accommodating extreme environments, such as one whose temperature ranges between 4xc2x0 K and 300xc2x0 K.
In an embodiment of the present invention, a uniform stiffness laminated composite shell assembly is provided which can be used in a tube suspension for a superconductive magnet. The laminated composite shell assembly includes a plurality of composite shells with substantially cylindrical configurations and predetermined axial directions. Each shell is made of composite layers of laminates having fibers (e.g., fibers wound into layers) with the fibers being oriented in a plurality of stacking sequences with reference to the axial direction of the shell. The fibers of the shells are made of graphite-epoxy material. The shells are concentrically assembled in a desired sequence with some of the shells being adapted to perform a structural load bearing function while others of the shells are adapted to perform a load transfer function. Some of adjacent ones of the shells in the desired sequence thereof are at least equal to or greater in axial length than others of the adjacent ones of the shells. The stacking sequences are selected to provide a ratio of axial-to-radial stiffness for extensional stiffness of approximately unity ensuring that each of the shells has uniform extension stiffness. The stacking sequences also are selected to provide a ratio of axial-to-radial stiffness for flexural stiffness of each of the shells of approximately unity ensuring that each of the shells has uniform flexural stiffness.